The selection of mice for innate immunity to Nematospiroides dubius was not specific. Mice bred asrefractory (R) to N. dibius infection were more refractory to primary infections with Nippostrongylus brasiliensis than other mice bred as liable (L) to infection with N. dubius. R and L, as well as randomly-bred (Rd) and inbred C3H mice were all immune to challenge infection with N. brasiliensis. Previous infections with N. brasiliensis failed to influence the course of N. dubius infections or the status of the selected mice as R. Rd and L to infection with N. bibius. R and L mice were equally susceptible to Mesocestoides corti infection but more resistant than Rd mice. R and L mice died sooner after infection with Salmonella typhimurium than Rd, although R survived longer than L mice.